Over the years the increased degree of development has resulted in an ever increasing demand for environment friendly and efficient energy storage systems. Among all the energy storage systems available, supercapacitors (SCs) are high in demand due to their distinctively high power density, reasonable energy density and longer cycle life [M. Winter and R. J. Brodd, Chem. Rev. 104, 4245 (2004); P. Simon and Y. Gogotsi, Nature Mater. 7, 845 (2008); J. R. Miller and P. Simon, Science, 321, 651 (2008)]. Based on the charge storage mechanism, the SCs can be categorized into two groups; electrical double layer capacitors (EDLC) and pseudo-capacitors (PC). In EDLCs the capacitance comes purely from the electrostatic charge accumulation at the electrode/electrolyte interface, which strongly depends on the surface area of the active material. Capacitors based on nanostructured carbon based materials having high surface area, fall under this category. On the other hand, PC or redox supercapacitors use fast and reversible active redox reactions for charge storage. Transition metal oxides (RuO2, Fe3O4 and MnO2) and electrically conducting polymers are its typical examples. Furthermore, these two mechanisms can simultaneously work together depending upon the nature of the active material.
Conducting polymers, specifically polypyrrole (PPy) has generated wide interest in the area of energy storage owing to its unique features such as high conductivity, environment friendliness, fast charge-discharge kinetics and low cost [L.-Z. Fan and J. Maier, Electrochem. Comm. 8, 937 (2006); H. An et al., J. Power Sources 195, 6964 (2010); B. C. Kim et al., J. Power Sources, 177, 665 (2008); I. Sultana et al., Electrochim. Acta, 60, 201 (2012); J. Wang et al, J. Power Sources, 163, 1120 (2007); J. Wang et al., Synth. Met., 161, 1141 (2011)]. Moreover, its characteristic redox doping-undoping process can be exploited in the charge storage systems, utilizing both the electrochemical double layer at the interface and pseudo-capacitive behavior. Nevertheless, the simultaneous occurrence of swelling and contraction with this doping-undoping process, adversely affects the materials stability, which ultimately affects its long term utilization.
To overcome the above mentioned problematic aspects, PPy has been used together with various carbon materials such as PPy/graphene [Y. Liu et al., Electrochim. Acta, 88, 519 (2013); X. Wang et al., Synth. Met. 162, 2349 (2012); J. Zhang and X. S. Zhao, J. Phys. Chem. C 116, 5420 (2012); CN 102051048 A], PPy/activated carbon [L. Wei et al., Adv. Funct. Mater. 22, 827 (2012)], PPy/carbon aerogel [H. An et al., J. Power Sources, 195, 6964 (2010)], PPy/single walled carbon nanotube [K. H. An et al., J. Electrochem. Soc. 149, A1058 (2002)], PPy/carbon nanofiber [L. F. Chen et al., ACS Nano, 6, 7092 (2012)] and PPy/carbon black [Yang et al., ACS Appl. Mater. Interf., 3, 1109 (2011)], etc. Conductive polymer/carbon material having high conductivity and long cycle life has been reported for various electrochemical storage devices [US008377546B2; US006842331B1; WO 2002019357 A8; CN 101302341 B; CN 1280318 C]. Although most of the porous materials exhibit large capacitance, the electrical conductivity deteriorates due to unavailability of conducting pathways or existence of oxygen containing functional groups [L. Li et al., J. Solid State Electrochem., 15, 175 (2010)] which largely limits the power capacity [L. F. Chen et al., ACS Nano, 6, 7092 (2012)]. Flexible electrode material for supercapacitor has shown a specific capacitance of ˜255 F/g [Chinese patent CN 102779648 A]. Recently [C. Yang et al., ACS Appl. Mater. Interf., 3, 1109 (2011)] have demonstrated that a specific capacitance of 366 F/g can be achieved in carbon black/PPy nanocomposites in 1.0 M NaNO3 electrolyte solution. Moreover, high conducting and thermally stable PPy can be synthesized using aromatic dopant anions [D. Lesueur and N. D. Alberola, Synth. Met. 88, 133 (1997); G. R. Mitchell et al., Synth. Met., 26, 247 (1988); I. Carrillo et al., Synth. Met. 162, 136 (2012)]. In a recent investigation [A. Kumar et al., J. Appl. Polym. Sci., 130, 434 (2013)] it has been found that aromatic dopant such as p-toluenesulfonate (pTS) is resistant to overoxidation and therefore can be used in electrode applications.
Hence to overcome the shortcomings of capacitance retention in the conducting polymer/carbon black composites, it is thought worthwhile to modify the polypyrrole-carbon (PPy/C) composites using aromatic dopant p-toluenesulfonate (pTS) [A. Kumar et al., J. Power Sources, 246, 800 (2014)] to obtain a high conducting PPy/C composites with substantial amount of thermal and electrochemical stability.